This invention relates to an improved instrument landing system, and more particularly relates to improvements in a type of landing system in which all installations sequentially radiate guidance pulses on the same frequency, the improvements permitting the ground installation of such a same-frequency landing system to be uniquely interrogated and hence identified by an approaching aircraft.
In cases where a ground installation of such a single frequency landing system is remotely located from other similar installations there is no need to be able to uniquely interrogate it and hence identify it to an approaching aircraft. However, in impacted geographic locations where there are multiple similar landing installations located relatively closely together it is necessary to provide means for uniquely interrogating and identifying one such same-frequency installation to the exclusion of others in the vicinity.
In conventional landing systems such as the present ILS and MLS systems, unique identification and signal exchanges between approaching aircraft and a particular ground installation are established by uniquely assigning different frequencies out of a band of frequencies to each of the various installations, and tuning the airborne units to the frequency of the selected installation. In my U.S. Pat. No. 4,429,312 entitled Independent Landing Monitoring System, a different type of identification of a same-frequency landing installation is discussed in which some of the signals transmitted to the aircraft are pulse encoded to identify that installation.
In this invention, ground installation identification is established by a unique interrogation technique, and in addition, the invention provides improved range data generation by the landing system which improved ranging is related to the presently disclosed unique technique of interrogation and station identification.
A very desirable characteristic for a landing system is the capability of providing range to touchdown, thereby to provide range data which serves three major purposes. A first purpose is providing range data for alerting the pilot of his proximity to touchdown. A second purpose is to provide means for automatically reducing the gain of the landing installation as the aircraft range to touchdown diminishes in order to maintain loop stability, often referred to as "course softening". A third purpose in providing range data which can be used, together with the elevational angular data provided by the landing system, to determine altitude above the runway during approach.
In the conventional ILS system, range to touchdown is generally provided by marker beacons on the ground at established distances from touchdown. These beacons radiate vertical fan shaped-beams through which the approaching aircraft passes. The range information thus acquired in the aircraft is used for pilot alerting and for "course softening" purposes.
In MLS and ILS practice, an alternative more accurate measurement of range is provided by conventional TACAN/DME interrogators which are carried by almost all aircraft. The airborne TACAN/DME equipment interrogates a DME beacon that is co-located with the MLS or ILS ground installation and receives therefrom a direct measurement of range using usual DME techniques. The DME units are assigned a band of 252 freqency channels in the 1000 MHz range, which band is separate from the 100-300 MHz frequency band allocated to ILS systems in current use or the 5000-5250 MHz band assigned to MLS.
For some landing applications, a very precise measurement of range is required, and for this purpose a Precision DME, usually referred to as PDME, is employed. The PDME is similar to the conventional DME, but uses faster rise time pulses to obtain higher precision. This PDME system imposes on aircraft, which have to use it in order to obtain a required very precise measurement of range, the additional burden of having installed on board appropriate PDME airborne equipment. Another technique for obtaining precision range in a landing system is provided by the teaching of my U.S. Pat. No. 4,429,312. Range is measured in that disclosure by having the weather radar interrogate the landing system ground installation and trigger the transmission of pulsed angular guidance signals. These pulsed replies are synchronous with the weather radar interrogations and are range tracked in a conventional manner to provide precision range in the aircraft. Range measurements of higher precision can be obtained by the use of fast rise time pulses.
Both of the above ways for identifying ground station installations, i.e. frequency selection or pulse group encoding, require additional adjustable cockpit controls for either tuning to the frequency of the ground installation, or for selecting the decodement of the signals radiated from that ground station. The measurement of range by means of marker beacons or DME equipment requires the installation of appropriate marker beacons or DME beacons with the landing system ground installation. The measurement of very precise range requires the addition of specialized PDME equipment, both air and ground. While the use of the weather radar to provide precision range, as taught in my U.S. Pat. No. 4,429,312, eliminates the need for added PDME equipment, not all aircraft carry a weather radar. Thus all conventional landing systems therefore have tended to require either added airborne equipment, or cockpit controls, or both, in order to achieve unique communication with a selected ground installation.
Considered broadly, a landing system does not inherently require the use of multiple different frequencies since operation at all installation sites is usually performed on a single frequency. Single-frequency operation is an advantage because if the actual landing guidance system can always operate on the same frequency for different sites, great simplification in terms of airborne equipment complexity is made possible since the receiver can be fixed-frequency. A technique to achieve both station selection and ranging data in a fixed frequency landing system, using only airborne equipment which is already installed in IFR aircraft, would eliminate the need to install in the aircraft any additional channel selection switches or decoder control switches, and hence would result in much simpler and lower cost airborne landing system installations.